(i) Field of the Invention
The present invention relates to a device for measuring not only the viscosity of a fluid in a flowing state but also characteristics such as the concentration of the fluid. More specifically, it relates to a device for measuring viscosity and a device for measuring characteristics of a fluid which can carry out the measurement with precision irrespective of the degree of characteristics such as the viscosity of the fluid.
(ii) Description of the Prior Art
Many products such as chemicals, foods, lubricating oils and car waxes have been manufactured, used or sold in the form of fluids, and for the control of each manufacturing process of these products and for the assurance of their performance, it is important to measure viscosities of these fluids.
Therefore, various methods and devices for measuring the viscosities have been heretofore known, and examples of such measurement methods include a capillary tube method, a rotation method and a falling-ball method.
However, in the capillary tube method, a sample is taken from a fluid to be actually measured, and the viscosity of the sample must be then measured. Thus, it is difficult to continuously measure and observe the viscosity of the fluid in a flowing state in a manufacturing process or the like. Particularly in the case of the fluid having thixotropy properties, there is a problem that it is extremely difficult to precisely measure its viscosity in a predetermined flowing state.
With the intention of solving this problem, there have been suggested a method and a device for measuring the viscosity in which a piezo-electric vibrator is utilized, and for example, Japanese Patent Application Laid-open Nos. 311250/1989, 213743/1990 and 189540/1991 have disclosed methods and devices for measuring the viscosity in which a piezo-electric element, particularly a quartz resonator is brought into contact with a fluid, and a change of resonance frequency or loss resistance at this time is utilized.
Furthermore, in Japanese Patent Application Laid-open No. 148040/1991, another viscosity measurement device has been disclosed in which a bimorph vibrator is vibrated at a predetermined vibration frequency in a fluid, and impedance at this time is then detected.
However, of such conventional viscosity measuring methods and devices, the techniques described in Japanese Patent Application Laid-open Nos. 311250/1989, 213743/1990 and 189540/1991 are poor in the precision of detection, because the circle of an admittance chart is not round at the actual viscosity measurement and thus the loss resistance cannot be consistently determined on the basis of the diameter of the circle, so that the reproducibility of the loss resistance tends to become insufficient. Furthermore, since admittance corresponding to frequency in the vicinity of resonance frequency changes in two modes of maximum and minimum, the precision of the detection is not considered to be sufficient. In addition, there is a problem that an electrode attached to the vibrator is directly brought into contact with the fluid, and so owing to the influence of dielectric constant of this fluid, the precise viscosity measurement cannot be accomplished.
On the other hand, in the viscosity measuring device described in Japanese Patent Application Laid-open No. 148040/1991, the bimorph vibrator has a relatively large amplitude, so that pulsation occurs in the fluid and this pulsation has a bad influence on the viscosity measurement. Moreover, as described above, the electrode of the vibrator is directly brought into contact with a fluid, and for this reason, there is another problem that the dielectric constant of the fluid has a bad influence on the viscosity measurement.
Furthermore, in such a conventional viscosity measuring device, for fluid having relatively high viscosity, the viscosity measurement can be done precisely to some extent. However, for fluid having relatively low viscosity, the piezo-electric element itself suffers mechanical resistance by vibration, and therefore the structure of the viscosity measuring device is largely restricted in order to permit the electrical constant of the piezo-electric element to change. Hence, the conventional viscosity measuring device has a problem that the viscosity cannot be measured with precision.